Joining zirconia with nickel-based superalloys for extreme applications by using a pressure-free high-temperature resistant adhesive

In order to meet the high demand for joining ceramic/superalloy composite structure in extreme environments, a novel high-temperature resistant adhesion technique was developed for joining ZrO₂ and Inconel 625 by applying an aluminum phosphate emulsion/zirconium sol based adhesive. With increasing temperature, a series of reactions occurred in adhesive, and its high-temperature bonding was attributed to the formation of a composite structure containing various ceramics and intermetallics. The adhesive after RT curing could find direct applications in extreme environments, and provide bonding strength no less than 2.5 MPa in the temperature range of RT-1100 °C. The bonding strength was higher than 4 MPa in the temperature range of 800–1000 °C, which was further attributed to the formation of an effective CTE-gradient relationship among ZrO₂, adhesive and Inconel 625, as well as the interfacial reactions between the two substrates. The work broadened the application of adhesion technique and brought new ideas for joining dissimilar engineering materials.     

On chemical and diffusional interactions between PCBN and superalloy Inconel 718: Imitational experiments

During a metal cutting process, chemical wear can become the dominant mechanism of tool degradation under the high temperatures and contact pressures that arise between the tool and the metal workpiece. This study focuses on the chemical and diffusional interactions between superalloy Inconel 718 and cubic boron nitride (cBN) tool material with and without TiC binder. It covers thermodynamic modeling and experimental tests in the pressure range of 0.1 Pa to 2.5 GPa at temperatures up to 1600 °C. The methods used include diffusion couples under both vacuum and high pressure, transmission electron microscopy (TEM) analysis and in-situ synchrotron observations. It is shown that cBN is prone to diffusional dissolution in the metal and to reactions with niobium, molybdenum, and chromium from Inconel 718. Adding TiC binder changes the overall degradation process because it is less susceptible to these interaction mechanisms.     

Microstructure and mechanical properties in the TLP joint of FeCoNiTiAl and Inconel 718 alloys using BNi2 filler

High entropy alloy(HEA) of Fe Co Ni Ti Al and Inconel 718 superalloy were firstly transient liquid phase(TLP) bonded by BNi2 filler due to the diffusion of Si and B in the filler to the base metals. The effects of bonding time on microstructure evolution and mechanical properties of the TLP joints were investigated.Owing to the complete isothermal solidification of the joints bonded for 30 min 120 min at 1100°C,no athermally solidified zones(ASZs) formed by eutectic phases were observed in the welded zone. Thus the TLP joints were only composed by the isothermally solidified zone(ISZ) and two diffusion affected zone(DAZ) adjacent to the dissimilar base metals and the negative effect of the ASZ on joint properties can be avoided. In addition, the increase of the bonding time can also make the Ti B2 borides precipitated in the DAZ near HEA and the brittle borides or carbides in the DAZ near IN718 alloy decrease and reduce the possibility of the stress concentration happened in the joints under loading. Therefore, the highest shear strength(632.1 MPa) of the TLP joints was obtained at 1100°C for 120 min, which was higher than that of the joint bonded for 30 min, 404.2 MPa. Furthermore, the extension of the bonding time made the fracture mechanism of the joint be transformed from the intergranular fracture to the transgranular fracture. However, as the brittle borides in the DAZ near IN718 can not be eliminated completely and refining of grains also happened in such region, all the TLP joints fractured inner the DAZ near IN718 alloy.     

The influence of the processing route on the fragmentation of (Nb,Ti)C stringers and its role on mechanical properties and hydrogen embrittlement of nickel based alloy 718

The nickel-base superalloy 718 is a precipitation hardened alloy widely used in the nuclear fuel assembly of pressurized water reactors (PWR). However, the alloy can experience failure due to hydrogen embrittlement (HE). The processing route can influence the microstructure of the material and, therefore, the HE degree. In particular, the size and distribution of the (Nb,Ti)C particles can be affected by the processing. In this regard, the objective of this work was to analyze the influence of cold and hot deformation processing routes on the development of the microstructure, and the consequences on mechanical properties and hydrogen embrittlement. Tensile samples were hydrogenated through gaseous charging and compared to non-hydrogenated samples. Characterization was performed via scanning and transmission electron microscopies, as well as electron backscattered diffraction. The processing was effective to promote significant variations in average grain size and length fraction of special Σ3ⁿ boundaries, as well as reduction of average (Nb,Ti)C particle size, being these changes more intense for the cold-rolled route. For the mechanical properties, on one side, the cold-rolled route presented the highest increase in ductility for non-hydrogenated samples, while, on the other side, had the highest degree of embrittlement under hydrogen. This dual behavior was attributed to the interaction of hydrogen with the (Nb,Ti)C particles and stringers and its ensuing influence on the fracture processes.     

Effect of deposition modes on electron beam directed energy deposited inconel 718

Inconel 718 thin walls were fabricated via electron beam directed energy deposition (EB-EDE) to investigated their microstructure and mechanical properties in terms of the deposition modes. Results revealed that the deposition modes had great effects on the microstructural evolution and thus influenced the mechanical properties. The layered nature and the fine dendrites were produced by the intermittent deposition, while the coarse and irregular cellular crystals were formed under the continuous deposition. The harmful Laves phase was precipitated under both deposition modes. The microhardness and tensile strength of the build-ups deposited intermittently were higher because there were fewer Laves phase. This work provided a new perspective to explain the microstructure differences of multi-layered components formed by EB-DED.     

Inconel 718合金的加工硬化行为

采用准静态拉伸试验方法研究Inconel718合金的加工硬化特性。结果表明,Inconel 718合金加工硬化规律基本遵循Hollomon公式。合金在退火态和冷拔态的应变硬化指数n 分别为0 476和0 15 3 ,拟合方程式的相关系数分别为0 89和0 99。室温条件下,Inconel 718合金塑性变形阶段的真应力-真应变关系可用Voce型公式描述,拟合方程式的相关系数大于0 99。     

固溶温度对激光增材制造Inconel 718合金组织和性能的影响

目的探索激光增材制造Inconel718高温合金最理想的固溶处理制度。方法利用激光增材制造技术制备了Inconel 718合金,通过组织观察(光学显微镜和扫描电镜)、能谱分析和维氏硬度测试等方法,研究了固溶温度对其组织、析出相及硬度的影响。结果不同固溶温度对Inconel 718的晶粒尺寸有很大影响。在固溶温度1000℃下保温1 h,沉积层开始出现再结晶现象。当固溶温度继续增加到1080℃时,与沉积态的组织相比,晶粒明显细化且再结晶过程基本完成。此外,不同固溶温度条件下,Inconel718的相析出和溶解行为也有所差异。固溶温度为940℃时,在未溶解的Laves相周围存在明显的δ相,当固溶温度继续提高时,δ相由于固溶作用而数量减少。另外,不同固溶温度处理后的合金显微硬度也表现出规律变化。当固溶温度为940℃时,试样硬度高于沉积态硬度,但是随着固溶温度持续升高,合金的显微硬度开始迅速下降并低于沉积态硬度,1050℃时保持稳定;当温度高于1150℃时,显微硬度继续迅速下降。结论激光增材制造Inconel718合金的热处理制度不同于铸造和锻造的热处理制度,其较为理想的固溶制度为1080～1150℃保温1 h。     

Functional encapsulation of laser melted Inconel 718 by Arc-PVD and HVOF for post compacting by hot isostatic pressing

In this study, the Selective Laser Melting (SLM) technology was used to manufacture flat specimens from Inconel 718 powder. The SLM process parameters have a major impact on the microstructure as well as on the mechanical properties of the fabricated specimens. Despite using optimized processing parameters, defects like pores cannot be completely avoided. These pores act as stress raisers and lead to premature crack initiation under cyclic loading, eventually reducing the fatigue strength of the material. Hot Isostatic Pressing (HIP) offers the possibility to eliminate the porosity and thus to increase the fatigue performance of the material. HIP combines high pressure and high temperature to produce materials with superior properties. Unfortunately, open porosity, i.e. open pores on the surface, can prevent full densification. In the present work, SLM flat specimens were encapsulated by means of Cathodic Arc Deposition (Arc-PVD) and High Velocity Oxygen Fuel Spraying (HVOF) to seal open pores. For this purpose, different encapsulation materials were investigated with a focus on materials offering additional functions such as an improved high temperature corrosion resistance or applicability as a bond coat for thermal barrier coatings.